1. Field of the Invention
The present invention relates to a liquid crystal display device of a negative display type having a light shielding layer formed therein.
2. Discussion of Background
Heretofore, a negative display wherein letters or patterns are displayed brightly on a dark display surface has been used in many cases for liquid crystal display devices used for the display portions of watches or meters for automobiles.
In a negative liquid crystal display device, no voltage is applied to the liquid crystal layer at the background area other than a display area and liquid crystal molecules are twisted at the background area, whereby liquid propagates along the twisted liquid crystal molecules, and a pair of polarizing films are disposed with their polarization axes being in parallel with each other so that no light is transmitted at the background area.
However, the negative liquid crystal display device had a problem in that a degree of polarization is decreased when light propagates in a twisted fashion, light having a certain color is transmitted to some extent, whereby there is caused insufficient contrast at the background area.
In order to resolve the above-mentioned problems, it has been proposed to use a negative type display device in which a light shielding layer is formed at the background area other than a display portion, and a polarizing film is disposed in the same manner as in the case of a positive liquid crystal display device so that light is transmitted at a portion where no voltage is applied to liquid crystal. Specifically, there is proposed a liquid crystal display device of a negative type such that a voltage lower than a threshold voltage or no voltage is applied to a predetermined display pattern area (hereinbelow referred to as a transparent segment) in the display portion so that liquid molecules are twisted to thereby transmit light, and a voltage higher than the threshold voltage is applied to the display portion other than a predetermined display pattern area (hereinbelow, referred to as a light shielding segment) so that liquid crystal molecules are vertically aligned to thereby shield light (Japanese Unexamined Patent Publication No. 162227/1985).
In the above-mentioned display device, since the light shielding segment vertically aligns to the substrate of the liquid crystal molecules, the direction normal to the substrate is coincident with the optical axis direction of the liquid crystal molecules, and they do not show double refraction property when viewed from that direction. Accordingly, there is obtainable substantially the same light shielding property as in the case that polarizing plates are bonded with their polarizing axes being crossed at a right angle, and it is possible to obtain a high ratio in contrast (contrast ratio) as high are at least 1,000.
When the above-mentioned display device is observed from the front, a display with a good contrast is obtainable. However, in this case, the figure of a viewer appears on the display because light reflects on the surface of the substrate, and the content of the display becomes illegible. In order to eliminate such disadvantage, there is proposed a method of inclining the substrate of the liquid crystal cell at an angle ranging from 10.degree. to 40.degree. with respect to the direction of observation which is determined to be at a right angle to the surface of the substrate. However, when the light shielding segment portion of the display device is viewed from an oblique direction, leakage of light is observed, the contrast ratio is decreased and legibility becomes poor. Further, the difference of brightness between the background area and the light shielding segment portion is great, and the difference of brightness between the light shielding segment portion and the transparent segment portion is small, whereby there is caused a problem that it is impossible to incline the substrate of the liquid crystal cell because the content of the display may be misread.